Invasive surgery is a very stressful event on the human body. This is especially true if the subject is elderly or is dealing with multiple conditions. After surgery, it is conventional wisdom to have the patient up on his or her feet as soon as possible as this can shorten the healing process. However, as can be imagined, the patient, after suffering the shock of surgery, is quite weak and mostly unable to stand or walk on their own.
To this end, physical therapists assist patients in taking their first tentative steps as early as a day after surgery. This proposition can be fraught with danger as the patient can fall and further injure themselves. Similarly, assisting the patient, who can become a dead weight when they fall, is not a simple matter for the therapist. Usually a therapist may require one or more assistants to assist a single patient regain their mobility.
To cut down on the dangers noted above as well as to reduce the manpower needed, devices which assist such patients are available. Such devices provide support for the patient as he or she regains mobility. Such devices follow the patient as he or she walks. If the patient should fall, these devices are designed to arrest the fall by catching the patient and providing compensating motion to counteract or stop the fall.
One such device is that disclosed in U.S. Pat. No. 7,803,125. This device provides pelvic support to the patient and, by sensing the patient's motion, the device can either move in the direction the patient is moving or, if the patient's motion is sudden, the device can compensate to arrest that sudden motion. Other, similar devices are, of course, also available.
However, one drawback for such devices is that they do not prevent the control system from instability. Because these control systems work by determining how much compensating force or torque is needed to address the system's needs (based on a reference model), the potential for exceeding the device's safety envelope exists. If the force or torque needed exceeds what the system can deliver, the components of the device may fail to function or may be pushed beyond their safety limits. Should the force or torque needed exceed what the system can deliver, then the motion of the device may become erratic, unpredictable, or unsafe, and the device may stop functioning. Due to this, the patient may be placed at risk of injury.
The stability of the interaction between the user and device is directly related to the stability of the control system. The factors that affect the stability of the control system include a variety of factors such as the human operator's and haptic device's dynamics, actuator bandwidth and saturation limits, and the position and admittance control loop parameters. Additionally, the stability of the user-device interaction is also influenced by factors related to the digital implementation of the position and admittance control loops (e.g., sampling rate, quantization, computation delay, and the use of zero-order-holds).
This search for stability directly leads to the design and performance of the system controller for haptic devices.
It is commonly assumed that if the gains of the position loop in the system controller are selected to be sufficiently large, then the haptic device dynamics may be assumed to be approximately linear. Furthermore, if the human dynamics are also assumed to be linear, then a variety of different robust stability measures may be used be to design linear position controllers that guarantee stable user-device interactions in the presence of parametric uncertainties in the estimates of the human operator's and haptic device's dynamics models.
The passivity formalism is also commonly used for designing admittance controllers for haptic devices. Roughly speaking, these controllers—or the conditions that the passivity formalism places upon their design—ensure that the combined user-device system does not generate any energy. Two approaches to passivity-based control dominate the literature. The first class of approaches is based on the idea of selective energy dissipation, and the second class of approaches consists of different techniques for selecting parameters of the control loop and reference model parameters (to satisfy the passivity condition.
Most of the controller design approaches described above assume that the haptic device is controlled using a linear position controller. However, simple linear position controllers may not be adequately robust to external disturbances and uncertainties due to modeling error. This fact has motivated the design of numerous different adaptive control algorithms that use standard Lyapunov stability arguments for designing stable position controllers. These approaches typically assume that the stability of the user-device interaction follows directly from the stability of the position control loop. Other robust admittance controllers based on internal model control and time-delay estimation, variable structure control, and iterative learning control have also been investigated, and similarly guarantee interaction stability via the stability of the position control loop. Moreover, some research has also been directed towards the design of model-free position controllers for admittance-controlled haptics that require little or no information about the robot's dynamic model.
In contrast to ad-hoc implementations based on manual tuning of linear position controllers, the advantages of the different types of controllers described above may be summarized as follows: adaptive controllers can estimate unknown device dynamics, provide robustness against modelling uncertainties, and guarantee interaction stability via the design of a stable position control loop; robust control-based approaches directly guarantee interaction stability in the presence of bounded uncertainties in the human, device, and environment dynamics; and, passivity-based approaches provide a conservative, but dependable guarantee on the interaction stability that is driven primarily by energy transfer considerations. While numerous different approaches for addressing the instability generated by external disturbances and modeling uncertainties exist in the literature, few approaches can account for the potential malfunction of the control system that can occur when during actuator saturation, i.e., when the system's actuators are incapable of generating the force or torque that is requested by the control law used in the control system.
There is therefore a need for systems, methods, and devices which address instabilities which may be generated by actuator saturation. As well, there is a need for similar devices or methods which minimize if not overcome the shortcomings of the prior art.